This invention relates to freeze-resistant, Perkins tube type heat exchangers, e.g. rotating Perkins tube type heat exchangers of the class disclosed in my earlier U.S. Pat. No. 5,123,479.
Perkins tubes are hermetically sealed enclosures wherein a part of the internal volume contains a working fluid and wherein all non-condensible gases are evacuated prior to closure. One of the first commercial applications of Perkins tubes was in ovens employed to bake bread. Over a century ago, Perkins tubes were made from Swedish iron, half-filled with water which was then brought to a boil, and the resulting steam drove out the air within the tube prior to crimping the open end and hermetically sealing it by welding. These tubes were installed by inclining one end into the wood or coal fire and permitting the tubes to pierce the oven wall and protrude into the bread-baking region. Thereby, the bread product would be free of wood and coal gas-contamination products. These ovens were located indoors and not ordinarily subject to freezing. If occasional freezing occurred, the tubes were short enough such that the contained water froze uniformly and no physical damage was experienced. This has been confirmed experimentally by freezing inclined (ordinary) tubes half filled with pure water.
It has been observed, however, that when the Perkins tube is partially finned externally, the freezing of water within the tube is not uniform. This obtains because, where the tube is finned externally, airside heat transfer is increased substantially from the outer tube-wall surface. This increased heat transfer causes the water within this portion of the Perkins tube to freeze first, causing an ice plug to form. This ice plug adheres to the inner tube wall surface and effectively forms a seal. Subsequently, water within the unfinned section, which is trapped by this plug, freezes; and, because the volume of ice is larger than the equivalent volume of water, the trapped water, upon freezing, expands the tube. Subsequent repetitions of this process cause eventual failure of the Perkins tube. This has been observed experimentally at the ends of the Perkins tube of the type used in the rotating heat exchanger described in the aforementioned U.S. Pat. No. 5,123,479.
Water is the preferred working fluid because it has been found to be compatible with copper and copper/nickel alloys up to its critical temperature; because it is nontoxic, because it is environmentally acceptable; because it is the most excellent heat transfer fluid in existence; because it is thermally stable to above its critical temperature; and because it will not fuel fires. It is unique among materials in that it expands upon freezing and, therein, it causes problems when it is used in pure form in finned heat exchangers, particularly of the type disclosed in U.S. Pat. No. 5,123,479 aforesaid.
It is noted that brines of calcium chloride and sodium chloride have been in use for many years as secondary coolants. Both calcium and sodium chloride brines are corrosive and the commonly used corrosion inhibitors have been sodium chromate in concentrations of 2 to 4 percent and pH control between 6.5 and 8.5. Because chromates are carcinogens, they have recently been replaced by sodium nitrite in concentration between 3% and 4% and pH control between 7.0 and 8.5. The 50% lethal concentration of sodium nitrate in humans is 0.3 weight percent in body tissue; and, therefore, their use in food applications is limited. organic inhibitors, which are stable at low brine temperatures, have replaced nitrites in systems employing brines in food applications. However, organic inhibitors are unstable at the high temperatures which are encountered in some Perkins tube usage.